This invention relates to a process of concentrating dilute sulfuric acid, particularly in a three-stage vacuum evaporation plant using forced circulation.
A process of concentrating dilute sulfuric acid is known in which the dilute sulfuric acid is fed to a first stage vacuum evaporation and concentrated sulfuric acid is withdrawn from a third stage vacuum evaporation. Steam and/or overhead vapor are used as a heating fluid in the vacuum evaporator plant using forced circulation. A pressure of 0.4 to 0.7 bar and a temperature of 80.degree. to 120.degree. C. are maintained in the evaporating vessel of the first stage. A pressure of 0.02 to 0.06 bar and a temperature of 50.degree. to 90.degree. C. are maintained in the evaporating vessel of the second stage. A pressure of 0.02 to 0.06 bar and a temperature of 80 to 120.degree. C. are maintained in the evaporating vessel of the third stage. Those overhead vapors, which are not used as a heating fluid, are condensed.
In the production of titanium dioxide by the sulfate process, dilute sulfuric acid, which contains metal sulfates, is formed in large quantities and must be concentrated before it can be recycled to the production process. As a rule, the dilute sulfuric acid formed in the production of titanium dioxide is evaporated by vacuum evaporation to a final concentration of 60 to 70% by weight (calculated without solids, i.e., without taking undissolved salts into account). Multistage vacuum evaporation plants using forced circulation are usually employed for that purpose. Published German Application 33 27 769 describes a process of treating dilute sulfuric acid in a process, which, in a preferred embodiment, includes a concentrating treatment in a three-stage vacuum evaporation plant using forced circulation. The dilute sulfuric acid, which is to be concentrated, is first fed to the first stage and is subjected to a first concentrating treatment therein. The dilute sulfuric acid, which has been concentrated, is subsequently concentrated further in each of the second and third stages. Concentrated sulfuric acid is withdrawn from the third stage.
A disadvantage of that process is that gypsum is formed in the first stage in large quantities and is deposited in the first stage and adversely affects the function of the first stage. These gypsum deposits can be removed only with difficulty and give rise to a need for expensive maintenance work, even after running the process for a short time so that the costs of the process are increased.